The invention is based on a short-arc high-pressure discharge lamp according to the preamble of claim 1. It involves, in particular, a short-arc high-pressure discharge lamp with a xenon fill, as is used in cinema projection.
The known xenon short-arc lamps for projection purposes were optimized for arc lengths and electrode geometries which are ideal for 35 to 70 mm film projection. The picture diagonals of these films lie in the range of between 28 and 60 mm. If such standard lamps are used in modern digital projection systems with DMD, DLP, LCD and D-ILA technology, then owing to the mismatch between the lamp and the optical system, a great deal of light is lost and does not reach the screen. This lost light is converted into heat in the projector and leads to additional problems. To date, it has been possible to resolve this problem only by a higher lamp power, which then requires greater outlay on cooling, an optimized mirror design, which places great demands on the accuracy and the simulation tasks, and additional double mirrors, which in turn entail cooling problems in the reflector volume.
It is an object of the present invention to provide a short-arc lamp with a xenon fill according to the preamble of claim 1, which permits optimum focusing of the light onto small cross sections of between 10 and 25 mm, corresponding to the diagonals of the integrators that are used in digital projection technologies (DMD, DLP, LCD and D-ILA).
This object is achieved by the characterizing features of claim 1. Particularly advantageous configurations can be found in the dependent claims. Further, the lamp is advantageously operated with a lamp current which satisfies the features of claim 6.
By setting the separation L in mm of the two mutually facing end sections of the anode and the cathode when the lamp is hot, according to the relationship 0.8xc3x97Pxe2x89xa6Lxe2x89xa61xc3x97P+1, where P is the lamp power in kW, optimum illumination of the picture window is achieved. With longer arc lengths, the efficiency of the system, i.e. the ratio of the output light flux to the incoming power, is significantly degraded. If the anode-cathode separation is shorter than in the relationship, then the life of the lamp is unacceptably reduced.
The stronger heating of the front surface of the anode (anode plateau) for shorter arcs also requires adaptation of the anode geometry. For instance, the diameter D of the anode in mm must satisfy the relationship Dxe2x89xa72.1xc3x97L+10, where L is the separation of the mutually facing end sections of the anode and the cathode in mm when the lamp is hot.
Advantageously, for optimum luminous efficiency with a long life, the frustoconical end section of the anode, which faces the cathode, should have a plateau AP with a diameter in mm that satisfies the relationship 1.8xc3x97Lxe2x88x921xe2x89xa6APxe2x89xa61.8xc3x97L+1, where L is again the separation of the mutually facing ends of the anode and the cathode in mm when hot. When the anode plateau diameter falls below this, strong erosion (cratering) on the anode plateau leads to a shorter life. In the case of an anode plateau that is larger than specified by the relationship, the system efficiency is degraded because of shadowing.
For optimum distribution of the light density throughout the life, the tip of the conical end section of the cathode is further advantageously designed as a hemisphere, wherein the radius R of the hemisphere in mm satisfies the relationship 0.12xc3x97P+0.1xe2x89xa6Rxe2x89xa60.12xc3x97P+0.5, with P being the lamp power in kW. Larger diameters of the hemisphere result in a lower light density, and smaller diameters lead to enhanced cathode burn-off.
Advantageously, the conical end section of the cathode has a vertex angle xcex1 of between 36 and 44xc2x0. Further, the frustoconical end section of the anode has, for optimum operation, a vertex angle xcex2 of between 90 and 105xc2x0. More pointed geometries lead to stronger burn-off of the electrode tips, while blunter geometries cause a high degree of shadowing in the projector.
For optimum operation with a sufficiently high efficiency (lumen/W), and an acceptable decrease in the light flux over the life of the lamp, the lamp should be operated, at a rated power P of between 0 and 5.5 kW, with a lamp current I in A of the relationship 22xc3x97P+38xe2x89xa6Ixe2x89xa622xc3x97P+65 and, at a rated power P of between 5.5 and 12 kW, with a lamp current I in A of the relationship 10xc3x97P+100xe2x89xa6Ixe2x89xa622xc3x97P+65. While weaker currents reduce the luminous efficiency in the system, the cathode erosion increases with stronger currents and the maintenance falls below acceptable values.